970
Inorg. Chem. 1996, 35, 970-973
Primitive Molecular Recognition Effects in Electron Transfer Processes: Modulation of ((Trimethylammonio)methyl)ferrocenium/ferrocene Self-Exchange Kinetics via Hydrophobic Encapsulation Roger M. Nielson,*,† L. Andrew Lyon, and Joseph T. Hupp* Department of Chemistry, Northwestern University, Evanston, Illinois 60208 ReceiVed May 12, 1995X
1H
NMR line broadening measurements show that the electron self-exchange rate constant for ((trimethylamino)methyl)ferrocenium/ferrocene (TMAFc2+/+) in D2O as solvent is decreased by ca. 20-50 fold in the presence of excess β-cyclodextrin. The rate effect is associated with the selective hydrophobic encapsulation of the ferrocene form of the redox couple (i.e., the ferrocenium form is not significantly encapsulated). Selective encapsulation leads to a coupling of electron transfer to host (cyclodextrin) transfer. Optical intervalence absorption measurements for a closely related mixed-valence system strongly suggest that the coupling decreases the self-exchange rate by increasing the thermal activation barriersan inference that is corroborated by activation parameter measurements. The barrier increase ultimately can be understood in terms of a redox asymmetry effect upon the isolated electron transfer event, where the overall exchange mechanism likely entails sequential electron and host transfer.
Introduction We and others have been exploring the consequences of molecular recognition and macrocyclic inclusion upon both optical and thermal electron transfer reactivity.1-3 Recently we discovered that hydrogen-bonding-based encapsulation of ammine ruthenium guests by crown ether hosts could induce large changes in electron self-exchange reactivity (RuIII/II)swhere the mechanism of rate modulation involved a coupling of electron transfer (ET) to subsequent host transfer.3d We reasoned that analogous redox reactivity effects might exist based on hydrophobic encapsulation: Here we report aqueous (D2O) selfexchange kinetics studies for ((trimethylammonio)methyl)ferrocenium/ferrocene (TMAFc2+/+) as a function of β-cyclodextrin (β-CD) encapsulation. While the exact encapsulation geometry is unknown, a prior investigation has yielded evidence for β-CD inclusion of both the unsubstituted cyclopentadiene and the methylene substituent of TMAFc+.1c
It should be noted that TMAFc2+/+ and β-cyclodextrin have been examined previously in ET cross reactions, and that a † Permanent address: Department of Chemistry, Southern Oregon State College, Ashland, OR 97520. X Abstract published in AdVance ACS Abstracts, February 1, 1996. (1) Representative kinetic studies: (a) Imonigie, J. A.; Macartney, D. H. Inorg. Chem. 1993, 32, 1007. (b) Imonigie, J. A.; Macartney, D. H. J. Chem. Soc., Dalton Trans. 1993, 1830. (c) Imonigie, J. A.; Macartney, D. H. Inorg. Chim. Acta 1994, 225, 51. (d) Johnson, M. D.; Reinsborough, V. C.; Ward, S. Inorg. Chem. 1992, 31, 1085.
0020-1669/96/1335-0970$12.00/0
variety of potential kinetic effects has been identified.1 We felt that the conceptual simplification offered by self-exchange processes would enable us to identify the most important of these effects. In any case, we find that (1) TMAFc+ is readily recognized and encapsulated by β-CD, (2) TMAFc2+, on the other hand, is not, (3) the resulting differential encapsulation substantially increases the barrier to electron transfer, and (4) the barrier increase and related factors attenuate the TMAFc2+/+ self-exchange rate by ca. 20- to 50-fold. Experimental Section Materials. β-Cyclodextrin septahydrate (Aldrich) was dried overnight at 100 °C. [TMAFc](I) was prepared from ((dimethylamino)methyl)ferrocene (Aldrich) via a literature method.4 Conversion to the BF4- salt was accomplished by adding aqueous [TMAFc](I) to saturated aqueous NaBF4. The resulting precipitate was washed with cold water, dried, and recrystallized from CH3CN/ether. TMAFc2+ was obtained from TMAFc+ by using benzoquinone in CH2Cl2 as a stoichiometric oxidant. Addition of HBF4 yielded solid [TMAFc](BF4)2 which was washed with ether. (Anal. Calcd (found) for C14H20NB2F8Fe: C: 38.95 (38.67); H: 4.67 (4.68); N: 3.24 (3.32).) Since solutions of TMAFc2+ are air sensitive, these were prepared in an argon atmosphere. Bis(ferrocenyl)acetylene (BFA) and [BFA](BF4) were synthesized via literature methods.5 Measurements. The electron self-exchange rate constant, k, was obtained via 1H NMR line broadening (Varian Gemini 300). In the (2) Representative thermodynamic studies: (a) Zhang, L.; Macias, A.; Lu, T.; Gordon, J. I.; Gokel, G. W.; Kaifer, A. E. J. Chem. Soc., Chem. Commun. 1993, 1017. (b) Isnin, R.; Salam, C.; Kaifer, A. E. J. Org. Chem. 1991, 37, 1939. (c) Strelets, V. V.; Mamedjarov, I. A.; Nefedova, M. N.; Pysnograeva, V. I.; Sokolov, V. I.; Pospisil, L.; Hanzlik, J. J. Electroanal. Chem. 1991, 310, 179. (d) Ando, I.; Ishimura, D.; Ujimoto, K.; Kurihara, H. Inorg. Chem. 1994, 33, 5010. (3) (a) Todd, M. D.; Yoon, D. I.; Hupp, J. T. Inorg. Chem. 1991, 30, 4685. (b) Curtis, J. C.; Roberts, J. A.; Blackbourn, R. L.; Dong, Y.; Massum, M.; Johnson, C. S.; Hupp, J. T. Inorg. Chem. 1991, 30, 3856. (c) Todd, M. D.; Dong, Y.; Horney, J.; Yoon, D. I.; Hupp, J. T. Inorg. Chem. 1993, 32, 2001. (d) Nielson, R. M.; Yoon, D. I.; Hupp, J. T. J. Am. Chem. Soc. 1995, 117, 9085. (4) Lindsay, J. K.; Hauser, C. R. J. Org. Chem. 1957, 22, 355. (5) (a) Rosenblum, M.; Brown, N.; Papenmeier, J.; Applebaum, M. J. J. Organomet. Chem. 1966, 6, 173. (b) Dong, T. Y.; Kambara, T.; Hendrickson, D. N. J. Am. Chem. Soc. 1986, 25, 4233. (c) Blackbourn, R. L.; Hupp, J. T. Chem. Phys. Lett. 1988, 150, 399.
© 1996 American Chemical Society
Primitive Molecular Recognition Effects
Inorganic Chemistry, Vol. 35, No. 4, 1996 971 occurring in the presence of β-CD, therefore, is
TMAFc+‚β-CD a TMAFc2+ + β-CD + e-
(3)
As shown in eq 4, the magnitude of the potential shift at a given cyclodextrin concentration can be used to calculate the TMAFc+‚β-CD association constant (K):9
E1/2 - Ef ) (RT/F) ln[β-CD] + (RT/2F) ln(Dox/Dred) + (RT/F) ln K (4)
Figure 1. Apparent formal potentials for TMAFc2+/+ vs log [β-CD]total. slow exchange limit6 (generally applicable here)
k ) π(WDP - WD)/[TMAFc2+]
(1)
where WDP is the full width at half-height for mixtures of TMAFc+ and TMAFc2+ and WD is the peak width for TMAFc+ only. The exchange kinetics for TMAFc2+/+ in 0.1 M NaClO4 or NaBF4 without β-CD, however, proved too rapid for eq 1 to be applied; k was instead determined via the fast exchange analysis shown in eq 2.7 In the
k ) 4πχPχD(∆ν)2/(WDP - χDWD - χPWP)Csum
(2)
equation, ∆ν is the contact shift in Hz, χD and χP are mole fractions of TMAFc+ and TMAFc2+, respectively, WP is the peak width for TMAFc2+ only, and Csum is the combined total molar concentration of TMAFc+ and TMAFc2+. Except as specified, the line broadening measurements were made at 20 ( 2 °C. Apparent formal potentials were determined by cyclic voltammetry by using a two-compartment cell containing a gold working electrode, a platinum counter electrode and a saturated (NaCl) calomel reference electrode (ssce). The electrolyte was 0.1 M NaBF4. Variable temperature measurements (seven temperatures between 2 and 30 °C) were performed by using a nonisothermal cell. Voltammograms were generated with a PAR 273 potentiostat. Fast voltammograms (5-11000 V/s) were obtained by coupling a signal generator (PAR 175 programmer) to the potentiostat. Signals in the higher sweep rate range were collected and averaged with a digital oscilloscope (LeCroy 9400; 125 MHz bandwidth). To minimize iR distortions in the fast experiments, positive feedback was employed, 10 µm diameter carbon disks were used as working electrodes (single compartment cell) and the electrolyte concentration was increased to 0.5 M. Near-infrared absorption measurements were made in D2O with an OLIS-modified Cary 14 spectrophotometer.
Results and Discussion Electrochemical Binding Studies. In the absence of β-CD, the observed formal potential (Ef) for TMAFc2+/+ is 376 mV. Figure 1 shows that cyclodextrin addition induces a Nernstian shift in the apparent formal potential (Ef).8 The sign of the shift indicates that cyclodextrin binding has occurred for the ferrocene form of the guest, but not for the ferrocenium form. The limiting slope at high cyclodextrin concentrations (60 mV) indicates that only a single host molecule is lost upon guest oxidation, in agreement with earlier reports.2 The redox process (6) Nielson, R. M.; McManis, G. E.; Weaver, M. J. J. Phys. Chem. 1989, 93, 4703. (7) See, for example: Chan, M. S.; Wahl, A. C. J. Phys. Chem. 1978, 82, 2542. (8) Reported formal potentials (Figure 1) will differ slightly from true formal potentials because of differences in diffusion coefficients for TMAFc2+ vs TMAFc+‚β-CD (see eq. 4).
In the equation, F is the Faraday constant and Dox and Dred are diffusion coefficients for the oxidized and reduced species, respectively. The diffusion term typically is negligible, but contributes about -12 mV here because host-guest association diminishes D by ca. 65%. From eq 3, K is 4900 ( 600 M-1 (at 22 °C). (For comparison, Imonigie and Macartney report K ) 4800 ( 600 M-1.)1c Extension of the electrochemical studies to both lower and higher temperatures yields, for eq 5, an enthalpy of binding of -5.9 kcal mol-1 and an entropy of binding of -3 eu.
TMAFc+ + β-CD a TMAFc+‚β-CD
(5)
In principle, the corresponding binding constant for the ferrocenium species (K ′) is available from the limiting potential shift at high cyclodextrin concentrations. In Figure 1, however, no evidence for high-concentration limiting behavior (i.e. loss of sensitivity of E1/2 to cyclodextrin concentration) is observed. This indicates that within the accessible host concentration range, no appreciable amount of TMAFc2+ is bound.10 Consequently, K ′ must be less than ∼20 M-1.11 Evans and co-workers have pointed out that in favorable circumstances cyclic voltammetry can also be used to determine the rate of host-guest dissociation (i.e. reverse of eq 5).12,13 The requirements are that (1) the host-guest assembly is electrochemically inactive in the potential range where the guest is oxidized, (2) electrochemical oxidation proceeds by initial dissociation of the guest:host assembly (i.e. a “CE” mechanism is followed), and (3) the dissociation rate constant is small in comparison to a composite quantity that includes the voltammetry sweep rate and the pseudo-first-order rate of host-guest association. When these requirements are met, the normally symmetrical voltammetry response will be characterized by an attenuated and flattened oxidation peak and an approximately normal reduction peak. We were unable to observe such effects here, even with sweep rates as high as 11 000 V/s and cyclodextrin concentrations as great as 10 mM. The negative results imply either that the dissociation rate constant is large (>2 × 107 s-1, assuming that a 50% attenuation in the oxidative voltammetry peak would have been observable) or that requirement 1 (above) is not met for this system. Consistent with either (9) It is important to note that the relevant concentration in eq 4 is the free host concentration, not the total concentration. The concentrations of free host were determined iteratively. (10) Additional (negative) evidence is provided by NMR measurements on β-CD. Significant shifts in two of the six proton resonances are induced by TMAFc+, but none are induced by TMAFc2+. (11) The solubility limit for β-CD was ∼23 mM (achieved by gently heating the host solution followed by spontaneous cooling to room temperature). Assuming that a 15 mV deviation from Nernstian behavior would be detectable at the highest host concentration, eq 1 and the data in Figure 1 yield the indicated upper limit estimate for K ′. (12) Matsue, T.; Evans, D. H.; Osa, T.; Kobayashi, N. J. Am. Chem. Soc. 1985, 107, 3411. (13) See also: Bard, A. J.; Faulkner, L. Electrochemical Methods: Fundamentals and Applications; John Wiley and Sons: New York, 1980 p 445.
972 Inorganic Chemistry, Vol. 35, No. 4, 1996
Figure 2. Logarithm of the TMAFc2+/+ self-exchange rate constant vs free β-cyclodextrin concentration in D2O as solvent. [TMAFc2+] ) (0.1-2.3) × 10-4 M-1. [TMAFc+] ) (1.59-2.11) × 10-3 M-1. Lines drawn are best fit lines to a logarithmic analog of eq 8: (0) without added electrolyte; (4) with 0.1 M NaClO4.
interpretation, Johnson has independently observed (via stoppedflow spectroscopy) that the dissociation rate constant exceeds 350 s-1.14 NMR Kinetics Studies. Figure 2 shows plots of the log of the TMAFc2+/+ self-exchange rate constant vs β-CD concentrationsboth with added electrolyte and without. Rates in the presence of electrolyte are significantly faster, as expected from work term considerations. More importantly, rates under both conditions are attenuated by as much as 20- to 50-fold following β-CD addition. In contrast to the behavior of ruthenium ammines and crowns,4d however, the reactivity does not recover with further additions of the host species. The electrochemical (thermodynamic) experiments point to the existence of two forms of TMAFc+, suggesting the possibility of a parallel reaction scheme involving free TMAFc+ (k°; eq 6) and encapsulated TMAFc+ (kβ; eq 7a) pathways.15 Fits of k°
TMAFc+ + TMAFc2+ {\} TMAFc2+ + TMAFc+ (6) k°
TMAFc+‚β-CD + TMAFc2+ {\} TMAFc2+‚β-CD + TMAFc+ (7a) TMAFc2+‚β-CD + TMAFc+ f TMAFc2+ + TMAFc+‚β-CD (7b) the observed rate constant to a parallel scheme (eq 8) are shown
k ) {k°[TMAFc+] + kβ[TMAFc+‚β-CD]}/{[TMAFc+] + [TMAFc+‚β-CD]} (8) in Figure 2,16 where the required concentrations of TMAFc+ and TMAFc+•β-CD have been obtained from the independently measured host-guest association constant. On this basis, the values of k° and kβ, without electrolyte, are 2.5 × 106 and 4.3 × 104 M-1 s-1, respectively. With electrolyte, they are 7 × (14) We thank Prof. M. D. Johnson of New Mexico State University for communicating these results to us prior to publication. (15) While eq 7b is written as a net host transfer reaction, presumably it involves sequential unimolecular dissociation and bimolecular association reaction components. (16) To avoid excessively weighting the fit toward k° (large) at the expense of accuracy in kβ (small), rate data were fit to a logrithmic form of eq 8.
Nielson et al.
Figure 3. Ln k vs 1/T: (0) [β-CD] ) 0; (O) [β-CD] ) 0.017 M.
106 and 3.0 × 105 M-1 s-1.17 The diminishing reactivity with increasing cyclodextrin concentration can be interpreted, therefore, as an effect arising from both the decreasing concentration of free TMAFc+ and the inherently attenuated reactivity of the encapsulated compound in comparison to the free species. It should be noted, however, that a finite reactivity for TMAFc+‚βCD is required in order to achieve reasonable fits to the data in Figure 2.18 Figure 3 shows an extension of the kinetics experiments where the temperature was varied for [β-CD] ) 0 and 17 mM. From the slopes, ∆H*(0) ) 4.0 kcal mol-1 and ∆H*(17) ) 7.3 kcal mol-1.19-21 While the parameters seem to suggest that enthalpic effects account for the decreased reactivity of the encapsulated species vs the free species, the comparison neglects complications from variations in K and, therefore, variations in the partitioning between reaction pathways 6 and 7. If the temperature dependence of K is considered, variable temperature estimates for kβ can be obtained and ∆H*(β) can be estimated as 1.5 ( 0.7 kcal mol-1.21 Intervalence Measurements. While activation parameters can offer insight into kinetic barrier effects, their interpretation (17) The directly measured k° values (i.e. rate constants in the absence of cyclodextrin) are 1.2 × 107 M-1 s-1 (without electrolyte) and 2.8 × 107 M-1 s-1 (with electrolyte). The disparity between measured and fit values for the former might be due to noncancelling systematic errors associated with the intercomparison of kinetic data collected in both the slow (eq 1) and fast (eq 2) line broadening regimes. (18) The analysis assumes rapid equilibration between bound and unbound reactant forms. An alternative scheme can be envisioned where guest: host dissociation is slow and direct reaction of TMAFc+‚β-CD with TMAFc2+ is neglected. At limiting high guest concentrations, k would correspond to the dissociation rate and the linewidth difference parameter, WDP - WD (eq 5), would become insensitive to the TMAFc2+ concentration. In contrast, the expected sensitivity is observed experimentally at all accessible host concentrations. (19) Because of solubility problems with NaClO4 solutions at low temperatures, activation parameters were obtained in NaBF4 solutions. Room temperature rate constants in the two electrolyte solutions were identical in the absence of cyclodextrin, but differed by ∼35% in the presence of cyclodextrin (17 mM). (20) ∆H* values were obtained from slopes of plots of ln k vs 1/T, where preexponentional factors were assumed (following conventional semiclassical electron transfer theory) to be temperature independent. Because of difficulties in observing sufficiently fast kinetics to render eq 2 applicable at low temperatures, values for k° were obtained only at 292 °C and higher. Subsequent estimation of kβ values at lower temperatures, therefore, required extrapolation of ln k° vs 1/T plots. (21) In the absence of electrolyte, ∆H*(0) ) 4.7 kcal mol-1, ∆H*(17) ) 7.9 kcal mol-1, and ∆H*(β) ) 3.8 kcal mol-1. The apparent entropies of activation, assuming a frequency factor of 1.9 × 1012 (the longitudinal relaxation time of D2O), are ∆S*(0) ) -10 eu, ∆S*(17) ) -7 eu, and ∆S*(β) ) -22 eu. With electrolyte the parameters are ∆S*(0) ) -10 eu, ∆S*(17) ) -6 eu, and ∆S*(β) ) -28 eu.
Primitive Molecular Recognition Effects
Inorganic Chemistry, Vol. 35, No. 4, 1996 973
Figure 4. Intervalence absorption energy maximum for BFA+ as a function of β-cyclodextrin concentration in D2O as solvent. First point ([β-CD] ) 0) estimated as described in ref 22.
without encapsulation, then an association constant of ∼10 M-1 for the ferrocenium site with β-CD can be inferred from the energy shifts. An alternate estimate based on the magnitude of the initial positive shift in Eop with β-CD addition yields K ≈ 25 M-1.23 In contrast to the activation parameter measurements, the optical measurements clearly imply that cyclodextrin-induced electron exchange inhibition is associated with a thermal barrier increase. In the context of reactions 6 and 7 and a stepwise electron transfer/crown transfer reaction sequence (see above), the observed optical effects translate into a thermal barrier effect of ca. 1.6 kcal mol-1 (∼550 cm-1 ≈ ∆Eop/2; see Figure 5). At ambient temperature, a barrier increase of this magnitude corresponds to a 14-fold decrease in rate. The remaining factor of 2-4 is presumably associated with “blocking effects” (nonadiabaticity effects) which have been implicated in other studies.1 To summarize, the available kinetic, thermodynamic and optical data indicate that ET reactivity diminution occurs when electron transfer becomes coupledsin a net sensesto host transfer:
TMAFc+‚β-CD + TMAFc2+ a TMAFc2+ + TMAFc+‚β-CD (10) While the overall reaction is energetically symmetrical (∆G ) 0), the isolated ET step (eq 7a) in a sequential electron transfer/ host transfer scheme is energetically unsymmetrical (Figure 5). The redox asymmetry induces a thermal barrier increase that can account for the majority of the observed 20- to 50-fold rate decrease caused by β-CD encapsulation. In principle, fast exchange kinetics should be recoverable (or partially recoverable) by additionally encapsulating the TMAFc2+ reactant (eq 11) and thereby removing the redox asymmetry. The failure
TMAFc+‚β-CD + TMAFc2+‚β-CD a TMAFc2+‚β-CD + TMAFc+‚β-CD (11)
Figure 5. Schematic representations of energy surfaces for optical and thermal electron transfer: top, without β-CD; bottom, with β-CD.
is often complicated by work term effects, ion pairing effects, residual entropy effects and other secondary phenomena. In principle, a more direct approach to barrier evaluation is to obtain optical intervalence absorption spectra. Measurements of this kind for reaction 9 (Figure 4) show that the optical
FcsCtCsFc+ f Fc+sCtCsFc β-CD‚FcsCtCsFc+ f β-CD‚Fc+CsCtCsFc
to observe rate recovery here is consistent with the thermodynamic inability to achieve TMAFc2+ encapsulation within the accessible β-CD concentration range. Acknowledgment. We gratefully acknowledge the National Science Foundation for support of our work (Grant CHE9303682 and Research Opportunity Award for R.M.N.) R.M.N. additionally thanks Southern Oregon State College for partial support for a sabbatical leave. J.T.H. gratefully acknowledges the Dreyfus Foundation for unrestricted support (TeacherScholar Award, 1991-1996).
(9a)
IC950569E
(9b)
(22) Because of the extreme insolubility of free BFA+ in water, the optical barrier (or intervalence absorption energy) in the absence of β-cyclodextrin has necessarily been estimated from optical and static dielectric constants for water, together with known correlations of Eop(BFA+) with dielectric properties in other solvents. The reported uncertainty represents two standard deviations in Eop in a previously published correlation of six solvents (Blackbourn, R. L.; Hupp, J. T. J. Phys. Chem. 1990, 94, 1788). (23) If ∆Eop arises exclusively from redox asymmetry effects3a,5c (i.e., if reorganizational effects can be neglected) then this quantity can be equated with -RT ln{K(ferrocene‚β-CD)/K(ferrocenium‚β-CD)}. If K(ferrocenium‚β-CD) equals 4800 M-1,1c then K(ferrocene‚β-CD) will equal 25 M-1.
barrier to ferrocenium/ferrocene electron exchange (Eop) increases by 1100 ( 200 cm-1 following β-CD association.22 The figure also reveals, however, a slight decrease in apparent barrier height in response to excess β-CD. The decrease is probably indicative of the onset of encapsulation at the ferrocenium site (i.e., binding of a second cyclodextrin). If, as in previous studies with crowns,3a,d the limiting intervalence energy with double encapsulation is approximately equal to that
